The AAA protein family (ATPases Associated with various cellular Activities, Kunau et al., Biochimie 75, 209-224, 1993) represents a large group of proteins that all share a highly conserved ATP binding domain of about 230 amino acids, exhibiting ATPase activity (Neuwald et al., Genome Research 9, 27-43, 1999; Vale, J. Cell Biol. 150, F13-F19, 2000). AAA proteins are widespread and have been characterised in Archaea, Eubacteria and all eukaryotic kingdoms. The AAA domains are required for protein functioning and are organised in hexameric rings that undergo conformational changes upon hydrolysis of ATP. This change in conformation, which is dependent on ATP hydrolysis, puts tension on bound proteins, and this mechanical activity allows unfolding of associated proteins, protein-protein dissociation etc. As a result, the AAA proteins play a role in different cellular processes, including cell cycle, organelle synthesis, mitochondrial functioning, vesicle transport, protein turnover, regulation of the cytoskeleton and intracellular motility. Thus, AAA proteins may represent a broad class of mechanoenzymes that have evolved unique ways of using a fundamentally similar conformational change in many different biological settings, but the basis of interaction with their target proteins is still a matter of speculation (Vale, 2000). So far, most efforts of research are focused on resolving the molecular structure and function of AAA ATPases, and consequently nothing is known about their role on a macroscopic level, such as plant growth or yield.
Given the ever-increasing world population, it remains a major goal of agricultural research to improve the efficiency of agriculture and to increase the diversity of plants in horticulture. Conventional means for crop and horticultural improvements utilise selective breeding techniques to identify plants having desirable characteristics. However, such selective breeding techniques have several drawbacks, namely that these techniques are typically labour intensive and result in plants that often contain heterogeneous genetic complements that may not always result in the desirable trait being passed on from parent plants. Advances in molecular biology have allowed mankind to manipulate the germplasm of animals and plants. Genetic engineering of plants entails the isolation and manipulation of genetic material (typically in the form of DNA or RNA) and the subsequent introduction of that genetic material into a plant. Such technology has led to the development of plants having various improved economic, agronomic or horticultural traits. A trait of particular economic interest is high yield.